Flexography is a type of relief printing that uses a flexographic plate of flexible rubber or resin, and a liquid printing substance. Currently, printing substrates (which is understood as any object on which printing is performed) that can be used for printing with this method include paper as well as cellophane and aluminum foil, and the like.
FIG. 10 illustrates a printing unit that constitutes a key component of a flexographic press. The printing unit includes an impression table 11 that supports a printing substrate 10, a printing plate 1 having a raised part 2, a plate cylinder 12, an anilox roll 16, a dispenser 18, and a doctor roll 15. Printing substance 17 such as ink is supplied to anilox roll 16 using dispenser 18. Anilox roll 16 and plate cylinder 12 are in the shape of cylindrical rolls that contact each other and are rotated in the directions indicated by arrows 48 and 46, respectively. Plate cylinder 12 includes on its perimeter surface printing plate 1 that has raised part 2 in a configuration that corresponds to the design to be printed. Raised part 2 and printing substrate 10 are disposed to be in contact with each other. Printing substance 17 is applied to raised part 2 by anilox roll 16 and then transferred to printing substrate 10. Printing substrate 10 is disposed on a main surface of impression table 11 and is moved in the direction indicated by arrow 47 as printing proceeds. The transferred design is defined by the top surface configuration of raised part 2. The substance that has been printed onto printing substrate 10 and has the configuration of raised part 2 is hereinafter referred to as a “printed substance”. In the present example, the printed substance 4 is in the shape of a frame.
The curved perimeter surface of anilox roll 16 is contacted by raised part 2 as well as doctor roll 15. Doctor roll 15 serves to uniformly spread printing substance 17 supplied by dispenser 18 over the perimeter surface of anilox roll 16. Thus, doctor roll 15 is disposed in contact with anilox roll 16 between the location where printing substance 17 is supplied and the location where it is in contact with raised part 2.
Alternatively, a plate-like doctor blade may be used that replaces, and functions similarly to, doctor roll 15. Further, a flexographic press may include a cylindrical fountain roll that replaces, and functions similarly to, dispenser 18 to supply printing substance 17 to anilox roll 16.
Conventionally, flexography has been used for printing characters and graphics onto packaging papers. However, as it may be employed in forming thin films, it is also used for other purposes than printing characters and graphics. For example, flexography may be used for forming alignment layers for a liquid crystal display, with a glass being the printing substrate and a polyimide thin film being printed on its surface.
Flat panel displays using e.g. a liquid crystal panel are employed in a variety of devices such as mobile phones, personal digital assistants, televisions and the like. The liquid crystal panel thereof has liquid crystal that is sealed between a pair of panel substrates spaced apart at a predetermined distance. A thermosetting or UV curing seal is used to bond the liquid crystal panels together along their periphery and to prevent the liquid crystal from leaking.
In recent years, a method of manufacturing liquid crystal panels called the dropping and panel-alignment method, or the dropping and filling method, has gained in popularity. The method preforms a frame-shaped seal on one of a pair of panel substrates and then drops a predetermined amount of liquid crystal within the frame. The panel substrate is then bonded together with the other panel substrate under a depressed atmosphere before retrieving them to the ambient atmosphere to produce a liquid crystal panel. The method allows the filling of liquid crystal and the bonding together of the two panel substrates simultaneously without leaving bubbles in the liquid crystal.
Conventional methods of forming a seal for the liquid crystal panel include the screen-printing method, the dispensing method and the like. In the screen printing, a screen mesh comes in contact with the surface of a printing substrate including a panel substrate and may scratch an alignment layer on the panel substrate, resulting in a decreased displaying quality. To prevent this, as disclosed in Japanese Patent Laying-Open No. 9-258194, methods have been proposed to separate the screen mesh from the printing substrate by inserting a spacer therebetween. This, however, requires emulsion to be thinly spread on the alignment layer, which may easily be punctuated, often causing the sealing material to be printed on the alignment layer. The dispensing method (see Japanese Patent Laying-Open No. 5-15818) uses a dispensing nozzle to print unicursally a frame using sealing material for each cell, which takes a long time. For example, it is inefficient when printing hundreds of small frames of sealing material on one panel substrate.
Consequently, methods have been developed for forming a seal without scratching the surface of the printing substrate while using flexography with improved productivity.
For a small film thickness of a seal in the dropping and panel-alignment method, when two opposed panel substrates are bonded together, part of the liquid may leak through a gap between the seal and the panel substrate prior to the entire periphery of the seal being in contact with the panel substrate to seal the liquid therein. Air may also enter the liquid crystal panel through a gap between the seal and the panel substrate upon retrieving the panel substrates from the depressed atmosphere to the ambient atmosphere. Accordingly, the pre-printed seal should have a thickness equal to or greater than 20 μm, and preferably a thickness in the range from 25 to 30 μms.
Flexography is suitable for producing a thin film of 0.01–1 μm. For a film thickness greater than several micrometers, attempts have been made to increase the amount of substance transferred from the anilox roll to the raised part of the printing plate by providing dot-like dimples on the raised part of the printing plate, as disclosed in Japanese Patent Laying-Open No. 10-217418. However, conventional flexography can stably produce a film thickness of around 10 μm, at most. The thickness of printing substance transferred to the printing substrate is hereinafter referred to as the “film thickness”. Trying to produce a greater film thickness requires correspondingly larger dot-like dimples on the raised part of the flexographic printing plate. However, printing substance is often insufficiently supplied from the anilox roll into the dot-like dimples, which leaves bubbles, resulting in bubbles in the printed substance after printing or in printed lines being partially narrower (a narrower portion of the printed line is hereinafter referred to as the “narrowness”).
The present invention solves the above-mentioned problems. An object of the present invention is to provide a relief printing plate and method of printing using the same, and an apparatus and method for manufacturing liquid crystal devices that allow printing for producing a film thickness greater than would be achieved by the conventional art without causing bubbles or narrownesses.